Apparatus for depth-selective raman spectroscopy

ABSTRACT

Apparatus and methods for detecting Raman spectral features non destructively from sub-surface regions of a diffusely scattering sample are disclosed. Incident radiation is supplied at one or more sample surface entry regions, and light is collected from one or more collection regions spaced from the entry regions. Raman features are detected in the collected light, and depth information is derived according to the entry-collection spacings.

FIELD OF THE INVENTION

This invention relates to the field of Raman spectroscopy and, inparticular, to apparatus for detecting Raman spectral featuresnon-destructively from sub-surface regions within a macroscopicdiffusely scattering sample.

Diffusely scattering media are frequently encountered in many analyticalapplications. Examples of such analytical applications include but arenot limited to monitoring of food products, colloids, polymers,catalysts, powders in general, coating technology and medicalapplications involving the probing of tissue and biochemicalidentification in medical studies and treatments. The samplesencountered in such analytical applications are often highlyheterogeneous and can be made of various layers each having a differentchemical make-up. Therefore, a major goal for analytical science is toprovide a method capable of determining the chemical composition ofsub-surface layers in a non-destructive way.

The benefits that can be derived from the analysis of chemicalcompositions can be illustrated using the example of non-invasive Ramanprobing of bones where signal quality from bone can be crucial inarriving at an accurate and correct diagnosis as to whether disease ispresent (see A. Carden and M. D. Morris, J. Biomed. Optics 5, 259(2000)). Conventional Raman signatures of bone collagen are masked byundesired Raman signals from overlaying tissue and so data on chemicalcomposition is generally obtained by means of a biopsy.

The separation of Raman spectral components derived from a surface layerof a sample and from a sub-surface layer of the same sample would behighly desirable in many analytical applications. This task is, however,hugely complicated due to an inability to form sharp optical images fromwithin turbid media of the overlying material by conventional opticalmethods such as confocal microscopy, which is only applicable typicallyto depths of the order of the transport length of-the scattering medium(the transport length describes the average distance photons travelbefore deviating significantly from their original direction ofpropagation). This length is typically around ten times the mean freescattering path of photons in the medium. For example, in biologicaltissue this corresponds to depths of around several hundred micrometres.

DESCRIPTION OF THE PRIOR ART

Infrared and Raman spectroscopies are known to provide a wealth ofinformation on the physio-chemical state of a wide range of biologicaltissues and fluids (see, for example U.S. Pat. No. 6,681,133 or WO01/52739). Unfortunately analysis by these methods has largely beenlimited to surface studies.

Elastically scattered photons have been used to probe beneath ascattering surface for compositional information. For example, B. B.Das, et al. in Rep. Prog. Phys. 60, 227 (1997) describes an approachusing temporal gating. This technique relies on the fact that it takes afinite time for light to penetrate a diffusely scattering medium.Scattering events will therefore occur later at lower depths, and somonitoring a scattered signal over time should, theoretically, provideinformation as to the nature of the scattering centres at progressivelygreater depths. The elastic scattering technique of Das et al. is,however, not directly applicable to Raman spectroscopy. Inelasticscattering of photons has a far smaller cross-section, resulting in amuch weaker signal. Furthermore the Raman signal is-far more susceptibleto interference from luminescence and in particular fluorescence whenanalysing tissue.

SUMMARY OF THE INVENTION

It is an object of this invention, therefore, to provide an analysistechnique based on Raman spectroscopy that is capable of extractingsub-surface chemical compositional information.

Accordingly the invention provides a method of measuring sub-surfaceRaman scattering through a diffusely scattering sample or medium, tothereby determine characteristics of the sub-surface region. In oneaspect the method comprises: (a) supplying incident radiation at anentry region on a surface of the sample, for example using a light probesupplied by a laser; (b) collecting, from a collection region on thesurface, light scattered within the sample, the collection region beingspaced from the entry region; and (c) detecting, in the collected light,one or more Raman features spectrally related to the incident radiation.

Just a single spacing between the entry and collection regions may berequired to obtain information about a sub-surface layer if sufficientprior knowledge is available, such as intensities of expected Ramanfeatures of the near surface region of the sample. However, bydetermining Raman spectral intensities at a variety of spacings moreaccurate information can be derived, for example using techniquesdescribed in more detail below.

A reference collection region may also be provided at substantially thesame place or overlapping with the entry region, for comparison with aspaced collection region. Advantageously, such a colocated referencecollection region may share common optics with the light sourcesupplying the incident radiation.

A variety of collection region and entry region geometries are possible.In one preferred embodiment the collection region forms an annulusaround the entry region. Multiple such annular collection regions, or asingle variable radius region may be used to derive more depth relateddata, and the arrangement may be reversed by placing a single fixed orvariable or multiple annular entry region around a central collectionregion. It should be noted that the annular regions may be broken orincomplete and will generally not include full optical coverage withinthese regions. For example, a circular single thickness array of closelypacked optical fibres might attain a 60% coverage.

A collection region is preferably spaced at least 1 mm and morepreferably at least 2 mm from an associated entry region. The depth of asub-surface layer to be studied will typically be similar to thisspacing distance, and the practical dimensions will depend on theoptical properties of the sample. The diameter or size of the entryregion will generally be less than the spacing of the entry andcollection regions, and the entry and collection regions are preferablynon-overlapping.

The present invention also provides a method of measuring a sub-surfaceRaman spectrum of a diffusely-scattering sample, the method comprisingthe steps of:

a) irradiating an entry region of the sample with a light probe;b) collecting light scattered by the sample; andc) spectrally separating at least a portion of the collected light todetect one or more Raman spectral features,wherein light scattered by the sample is collected from a plurality ofspatial locations or collection regions on the surface of the sample,each spatial location being at a different distance from the point ofirradiation, at least a portion of the light collected at each spatiallocation being separately spectrally dispersed to form a plurality ofRaman spectra and wherein the method further includes the step of:d) analysing the plurality of Raman spectra to extract information onthe Raman spectrum of a sub-surface region of the sample.

Thus, spectroscopic information is obtained non destructively that canbe interpreted to establish the nature and composition of a diffuselyscattering sample below a surface layer. The present invention asdefined by the methods outlined above effectively implements a form ofspatial gating of the Raman signal obtained from the sample to isolatethe Raman signal from a sub-surface layer which has a differentcomposition to that of the surface layer. This method is referred toherein as Spatially Offset Raman Spectroscopy (SORS).

With the present invention for samples having one or more differentchemical compositions at differing depths within the sample, thecollection of Raman spectra from regions spatially offset, by differentamounts, from the point of incidence of the probe laser beam results ina series of spectra (two or more spectra) each spectra including Ramansignals emanating from different depths within the sample. The series ofspectra taken contain different relative contributions of the Ramansignals generated from the sample surface layer and the samplesub-surface layers. In collecting the data series, as the signalcollection point is moved away from the point of incidence of the probelaser beam, the contribution of the surface layer signal diminishes muchfaster than for signals generated by different compositions at deeperlayers within the bulk of the sample. This enables the contribution ofdeeper, sub-surface layers to be extracted either directly or byapplying numerical processing to the collected spectral set for a higherdegree of separation (e.g. multivariate data analysis or scaledsubtraction of spectra from each other).

In a preferred embodiment two or more Raman spectra are collected andare analysed using a scaled subtraction, the Raman spectrum collectedfrom or at a distance closest to the point of irradiation beingsubtracted from the Raman spectrum collected further from the point ofirradiation, whereby features of the Raman spectrum for a sub-layer ofthe sample are identified.

In a further alternative, where the Raman spectrum for the chemicalcomposition of the surface of the sample is known, the collected Ramanspectra are analysed by scaled subtraction of the known Raman spectrumfrom the Raman spectra of the collected light.

In an alternative preferred embodiment at least twenty Raman spectra arecollected at different distances from the point of irradiation and theplurality of Raman spectra are analysed-using multivariate dataanalysis. Principal component analysis may be used as the multivariatedata analysis.

A preferred feature of the present invention is irradiation of thesample at two or more different wavelengths, where the collected lightis a combination of a Raman spectrum and fluorescence, so that the Ramanspectrum can be extracted from the collected light.

At least one of the sample, the collection optics and the point ofirradiation may be moved relative to the others to enable the collectionof Raman spectra at different distances from the point of irradiation. Amovable stage may be provided on which the sample is mounted and theprobe beam is arranged to track the movement of the sample whereby thesample is moved relative to fixed collection optics for the collectionof scattered light at different distances from the point of irradiation.

The scattered light may be collected from point regions at differentdistances from the point of irradiation or the scattered light may becollected from a plurality of substantially parallel lines substantiallytransverse to the distance as measured from the point of irradiation.

Alternatively, the probe beam may be supplied using optical fibres andthe scattered light may be collected using optical fibres arranged in aplurality of concentric circles around the probe beam optical fibreswhereby the scattered light is collected in concentric rings atdiffering radii from the point of irradiation.

Ideally, the light probe is at >200 nm and <2000 nm and may be generatedby one or more quasi-monochromatic lasers or a diode laser which istunable, for example with respect to temperature.

In an alternative aspect the present invention provides apparatus forselectively measuring Raman spectra generated at different depths withina diffusely-scattering medium, the apparatus comprising: a light sourcefor irradiating a sample with a probe beam; collection optics forcollecting light scattered by the sample and passing it to aspectrometer; detection means for detecting light dispersed by thespectrometer, wherein the apparatus is adapted for scattered light to becollected at a plurality of spatial locations on the surface of thesample, each spatial location being at a different distance from thepoint of irradiation and at least a portion of the light collected ateach spatial location being separately spectrally dispersed by thespectrometer to form a plurality of Raman spectra and wherein theapparatus further includes an analyser for identifying features specificto the Raman spectrum of a sub-layer of the sample from the plurality ofRaman spectra.

The apparatus preferably includes a movable stage for relative movementof at least one of the sample, the collection optics and the point ofirradiation to enable the collection of Raman spectra at differentdistances from the point of irradiation. Alternatively, the collectionoptics may comprise optical fibres arranged in a plurality of concentriccircles around the probe beam.

The light source may consist of one or more quasi-monochromatic lasersor a diode laser which are tunable, for example with-respect totemperature.

In a further alternative aspect the present invention provides a methodof diagnosis comprising collecting from a sample, consisting of asurface region of an overlying tissue and a sub-layer region of a deeptissue which is different to the overlying tissue, one or more Ramanspectra using the method as described above.

Preferably one or more features specific to the Raman spectrum of thesub-layer region of the sample are identified in the one or morecollected Raman spectra and are compared with those obtained from ahealthy control specimen.

Embodiments of the invention will now be described by way of exampleonly and with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates principles of the invention, in which illumination bysource 10 leads to a Raman scattering 16 in a sub-surface region ofsample 14, the Raman photon being received at collector 18;

FIGS. 2 a to 2 c illustrate various sample layer arrangements;

FIGS. 3 a to 3 c illustrate various entry and collection regionarrangements;

FIG. 4 shows an arrangement for varying the diameter of an annularcollection region using an optical arrangement 50, 54;

FIG. 5 illustrates the use of mirrors 60 to enhance the collection ofRaman photons;

FIG. 6 shows an optical head 70 for coupling to a spectral detector 22;

FIGS. 7 a and 7 b show plan details of the optical head and connector ofFIG. 6;

FIG. 8 illustrates schematically analysis apparatus in accordance withthe present invention set up to extract Raman spectra generated beneatha surface layer of a sample;

FIG. 9 illustrates a point collection geometry for collection ofspatially offset Raman spectra in accordance with the present invention;

FIG. 10 illustrates a concentric circle collection geometry forcollection of spatially offset Raman spectra in accordance with thepresent invention;

FIG. 11 shows a series of Raman spectra for a two layer sample generatedat different offsets using the analysis apparatus of the presentinvention;

FIG. 12 illustrates the dependence on offset distance of the absoluteintensities of the Raman spectra for the sample of FIG. 11;

FIG. 13 illustrates the ratio of the Raman spectra of FIG. 12 withrespect to offset distance;

FIG. 14 shows a series of Raman spectra for the same two layer samplescaled to the same height of trans-stilbene bands;

FIG. 15 illustrates the PMMA contributions within the individual spectraof FIG. 14;

FIG. 16 shows, for the same sample, the relative ratio of atrans-stilbene Raman signal in comparison with fluorescence originatingfrom the PMMA layer as a function of the spatial collection offset;

FIG. 17 shows the results of a PCA analysis of a series of Raman spectrafor the same sample obtained using the analysis apparatus in accordancewith the present invention; and

FIG. 18 shows the results of a simple subtraction process with respectto the same sample using Raman spectra obtained by the analysis methodin accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1 an embodiment of the invention is shown inoperation, in schematic cross section. A light source 10, incorporatingor supplied by laser 9, is used to irradiate a localised entry region 11of a surface 12 of a sample 14. The incident radiation from the lightsource is scattered diffusely through the sample. Some of the radiationmay be absorbed by the sample, some may give rise to optical emissionsfor example by fluorescence, and some re-emerges unchanged through thesample surface 12.

A small proportion of the photons of the incident radiation areinelastically scattered giving rise to Raman photons, for example asillustrated by Raman event 16. The Raman photons in turn are diffusivelyscattered through the sample. Some may be absorbed, for example givingrise to fluorescence, but some emerge unchanged through the surface 12to be collected at collector 18. The likelihood of a Raman photonundergoing a second Raman event is very small.

The collected light is analysed, for example using filters or aspectrometer, and a suitable sensor in detector 22, and the determinedRaman spectra or spectral features are used further in analyzer 23,which is typically a computer. The detector may use a fourier transformrather than a conventional dispersive spectroscopic technique.

Typically, most Raman photons will be generated close to the lightsource 10, where the incident radiation is most intense. These Ramanphotons can best be detected by collecting light at the light source 10,for example by using optics common with the light source. As distancefrom the light source increases, however, the intensity of Raman photonsoriginating near the light source falls away more quickly than theintensity of Raman photons originating further away from the lightsource, especially from deeper within the sample. Preferential samplingof Raman photons from deeper within the sample can therefore be achievedby spacing the location at which light is collected from the location atwhich the sample is illuminated, and an analysis of how the detectedspectral features change with spacing can provide more detailedsub-surface information.

In FIG. 1 Raman event 16 occurs in a subsurface layer 20. The spacing dbetween the light source 10 and the collector 18, or equivalentlybetween an entry region 11 and a collection region 19 can be adjusted toselect for a particular depth. In preferred embodiments, however, lightis collected at a range of two or more spacings d, and an analyzer 23 isused to infer depth dependent characteristics of the sample from theRaman features of the collected and analyzed light for different valuesof d, which are spectrally analyzed by analyzer 22. One of the spacingscould be at, or very close to the entry region.

For example, the analyzer may preferentially select (or reject) theRaman features for a particular depth or range of depths by combiningthe Raman features for different spacings. Some techniques for this,including scaled subtraction and PCA are discussed below.

In FIG. 1 the sample 14 displays a non-abrupt boundary between thesurface region and sublayer 20. In FIGS. 2 a to 2 c some other sampleconfigurations are shown. In FIG. 2 a there is an abrupt change fromsurface layer 30 to deep layer 32, and deep layer 32 may be diffuselyscattering, or partly or completely opaque, with Raman photonsrepresentative of layer 32 being generated at the layer interface. InFIG. 2 b the surface layer 30 and the deep layer 32 are separated by afurther transparent or semi transparent layer 34 which may be, forexample, a space filled with a liquid or gas. In FIG. 2 c a more complexsample is shown, in which graduated or abrupt sublayers 36 and 38 areembedded beneath or within the surface layer 30.

The incident irradiation and collection of light at a single, atmultiple or at a variable spacing can be achieved using a variety ofgeometries. In FIG. 3 a there is a single illumination or entry region40 on the sample surface. Spaced from this illumination region is eithera single collection point or region 42, or multiple regions as indicatedby the broken lines. Alternatively the single collection region, orequivalently the illumination region may be moved to provide a variablespacing.

In FIG. 3 b the single illumination region 40 is surrounded by anannular collection region 44, or by multiple or a variable radiusannular collection region as indicated by the broken lines. Instead ofan annular collection region, a broken annulus or multiple separateregions at similar distances from the point of illumination could beused.

In FIG. 3 c an annular illumination region 46 and central collectionregion 48 are used, thereby reducing the localised intensity of incidentradiation required to generate a given number of Raman photons. Theannulus may be varied in radius or be provided as multiple annuli havinga range of radii, and a broken annulus of multiple separate illuminationregions distributed at similar distances from the central point ofcollection may be used.

Generally, it is beneficial to collect light, or to provide incidentradiation at as large a proportion of an entry or collection region aspossible. However, in practical embodiments the coverage may be limited.For example, in arranging cylindrical optical fibres in an annulus acoverage of 10% or more may be adequate, but 25% or more would bepreferred and 60% or more may be possible.

In simplistic embodiments a single entry region may be provided by asingle optical fibre brought close to the sample surface, and multiplecollection regions may be provided by a linear array of collectionfibres. Optical fibres may be similarly used to provide annular andother configurations of single and multiple fixed spacings and variousmechanical arrangements may be used to provide variable spacings.

To provide a variable radius entry region or collection region anoptical arrangement such as that illustrated in FIG. 4 may also be used.Optics 50 located between the sample and the collector and/orsample-to-detector distance is adjustable to direct light from differentparts of the sample surface onto collector 18 which is concentric withthe light source 10. In this arrangement, a lens arrangement (and/or theillumination source and Raman collector detector position) which can betranslated in an axial direction 52 by an optics drive 54 directs lightfrom an annular region of varying radius onto the collector, but otherconfigurations are also envisaged.

A further aspect, which may be used with any of the arrangementsdiscussed above, is illustrated in FIG. 5. One or more mirror elements60 are presented to the sample surface. When either incident or Ramanradiation emerges from the sample away from the collector 18, thesemirror elements redirect the emerging radiation back into the sample.This increases the intensity of incident radiation and so the generationof Raman photons within the sample, and also increases the proportion ofRaman photons received at the collector 18. The mirror elements arepreferably absent from the surface adjacent to the light source 10 orentry region, and adjacent to the collection regions.

In alternative embodiments non-imaging optics, such as those describedin Applied Optics vol 35 p 758, may be used to achieve higher collectionefficiency by use of a mask placed directly onto the sample, or placedin an image plane if other imaging optics are also used. The mask blocksappropriate areas of the sample to collect signal from a desired spatialoffset only. The masking is preferably synchronised with a detector suchas a charge coupled device such that sequential readings from thedetector relate to masks providing light collected from correspondinglysequential spacings between the illumination and collection regions. Themasking could be mechanical and could also be performed between imagingoptics and a non-imaging type detector.

FIG. 6 illustrates a practical embodiment of the invention comprising anoptical head 70 coupled by an optical fibre bundle 72 to analyser 22.Detail of the optical head 70 is shown in the plan schematic view ofFIG. 7 a which is not to scale. A bundle of light source optical fibres74 terminate in the central region of the head. These light sourcefibres are embedded in a filler 76 such as epoxy, and surrounded by anannular spacer element 78. Collection optical fibres 80 terminate anannular region surrounding the spacer element, again embedded in afiller, and surrounded by an external casing. This arrangement may beadapted to included the various mirror and optical arrangementsdiscussed above.

In this particular embodiment each optical fibre has a core of 200 μmdiameter and a cladding bringing the fibre thickness to 230 μm. Theinner bundle consists of seven light source optical fibres 74, and theouter bundle consists of 26 collection optical fibres 80. The spacer 78is sized to space the collection fibres 80 about 3 mm from the centre ofthe head, and the terminations of the collection fibres are distributedapproximately evenly in an annulus of constant radius about this centre.The collection fibres should be suitable for carrying out near infra redRaman work, and may be made of silica.

The illumination and collection optical fibres terminate, about 100 cmdistant from the optical head, in a connector illustrated schematicallyin FIG. 7 b. The connector presents the six illumination and twenty sixcollection fibres for coupling into the analyzer 22 of FIG. 6, whichincorporates a light source illumination quasi-monochromatic laseroperating at 827 nm and a Kaiser Holospec optical analyser.

A schematic diagram of another spatial gating analysis apparatus foridentifying depth specific Raman spectra is shown in FIG. 8. Thefeatures and variations already described above may be applied here asappropriate, such as the various collection geometries, and vice versa.The apparatus generally comprises a laser 101, Raman detection apparatus102, 103 and an analyser 104. The probe beam 105 of the apparatus isgenerated using a quasi-monochromatic laser such as a single-line cwargon ion laser operating at 514 nm, (in the case of tissue analysis 827nm would be preferred to avoid fluorescence background), with 12 mWpower which is directed using conventional optics at a sample. Thesample has a surface layer 106 and a deeper layer 107 of a differentchemical composition to that of the surface layer, and may be mounted ona stage. With this apparatus the laser plasma lines were blocked using aPellin-Broca prism (not illustrated). The apparatus includes a 1 m focallength lens 108 for weakly focusing the laser beam onto the sample to aspot diameter of 300 μm and at normal incidence. Raman light produced asa result of the irradiation of the sample is collected in backscatteringgeometry using a 2″ diameter collection lens 109 with f-number ˜1 and isimaged with the lens 109 onto the slit of a spectrometer 102, which ispart of the Raman detection apparatus, with a magnification of 2.5. Aconventional

imaging spectrometer 102 (for example a Spex Triplemate™ with f-number6.3) is preferably used to disperse the Raman light and image the Ramanlight onto a CCD camera 103. The camera 103 is preferably a liquidnitrogen cooled back-illuminated deep depletion CCD camera (for exampleAndor, DU420-BU2 (250 nm) 1024×255 active pixels). The CCDquantum-efficiency of such a camera in the region of Raman spectra isaround 65% and has a pixel size of 26×26 μm. The final stage slit widthof the 25 spectrometer 102 was set to 120 μm. The CCD was binnedvertically across 20 pixels to maintain the spatial selectivity on thecollection side.

The sample 106, 107 was mounted on an x-y-z micro-positioning stage 110which includes a controlled drive (not illustrated) which moves thestage (vertically in FIG. 8) together with the final optics to keep theincidence point of the laser beam fixed on the sample with respect tothe sample. In this configuration, the Raman detection apparatus 102,103 always collects back scattered Raman shifted photons from a fixedimaging zone in space and the sample is scanned across this imaging zonewhilst the pump beam incidence point remains fixed in its position onthe surface of the sample. A filter (not illustrated) may also be usedto block any residual elastically scattered probe laser light fromreaching the spectrometer 102. The SORS apparatus described above may bedeployed using a point collection laterally offset from the point ofprobe beam incidence (FIG. 9). Alternatively, a movable stage or othermovement control means may be used for achieving relative movementbetween one or more of the sample, point of irradiation and the Ramandetection apparatus.

Raman spectra using apparatus similar to that described above werecollected for a test sample in which the first layer 106 consisted of a1 mm optical path cuvette of 1 cm width and ˜4 cm height, with 300 μmcustom made fused silica front and back windows, filled with PMMA(poly(methyl methacrylate)) spheres of ˜20 μm diameter. The spheres wereloosely packed in the cell using mechanical tapping on the cell duringfilling to eliminate any larger voids. This first layer was followed bya second layer 107 consisting of another cell of 2 mm optical pathfilled with trans-stilbene fine powder ground using a mortar and pestle.The cuvettes were employed in order to provide a simple method of samplehandling and are not an essential feature of the apparatus.

With the probe laser beam incident on the sample positioned with thefirst layer 106 uppermost, spatially offset Raman spectra using the SORSmethod described herein were collected using a basic point collectiongeometry in which collection is from the points laterally displaced fromthe probe beam's incidence point (FIG. 9). The point of collectiongeometry as illustrated in FIG. 9 represents the simplest implementationof the method of the present invention. On the other hand, theconcentric circle geometry illustrated in FIG. 10, which does notrequire the use of an x-y positioning stage, advantageously yields muchhigher collection efficiency but involves the use of optical fibres toimage the individual circles at different heights on the spectrometerslit enabling their imaging after dispersion on the CCD 103 ontoseparate horizontal strips with the vertical position of the spectra onthe CCD corresponding to a given offset collection distance on thesample surface with respect to the probe beam's incidence point. The useof a fiber optic bundle for the collection of Raman spectra is describedin an article by Jiaying Ma and Dor Ben-Amotz entitled “RapidMicro-Raman Imaging using Fiber-Bundle Image Compression” AppliedSpectroscopy Vol. 51, No. 12, 1997 the contents of which is incorporatedherein by reference.

It will, of course, be apparent that further alternative collectiongeometries could be employed whilst still achieving spatially offsetRaman spectra collection in accordance with the present invention.

Additionally, with no sample illumination, an “above the sample” Ramanspectrum may be collected which represents background and apparatusnoise. This “above the sample” Raman spectrum can then be subtractedfrom the set of Raman spectra to remove noise from the spectra.

When taking Raman spectra using the resonance Raman technique, wherebythe wavelength of the incident probe beam is tuned to match chromophoresof the material or materials being investigated, the Raman signaturesmay be swamped by fluorescence (luminescence) generated from electronicexcitation. For example, fluorescence will be stimulated in roomtemperature studies of bone, but phosphorescence is more likely incolder samples. Similarly, Raman probing of metallic systems will oftenstimulate room temperature phosphorescence.

In such cases the Raman spectra can be recovered using the SORS methodat two or more laser wavelengths. This relies upon the fact that thespectral profile of a fluorescent background is not normally dependenton the excitation wavelength whereas the Raman spectrum is dependent onthe excitation wavelength. Hence, spectra collected at the same spatialdistance from the point of illumination at two or more differentwavelengths of -irradiation may be subtracted from each other to give aderivative type plot of where the Raman bands are and this can bemathematically processed to give a truer looking Raman spectrum. Thistechnique for identifying Raman bands is described in an article by S.E. J. Bell, E. S. O. Bourguignon and A. C. Dennis entitled “Subtractedshifted Raman spectroscopy (SSRS) method” Analyst, 1998, 123, 1729-1734.This technique is also referred to as the Shifted Excitation RamanDifference technique (SERD) as described in a paper of the same name byP. Matousek, M. Towrie and A. W. Parker I J. Raman Spec., 33, 128-242(2002) the contents of which is incorporated herein by reference.

The two or more wavelengths of incident irradiation may be generated bymeans of separate lasers or by means of a single laser, such as a diodelaser, the output of which is varied for example through temperaturetuning. The difference in wavelength required is generally about halfthe width of the Raman bands, typically approximately 5-10 cm⁻¹.

A set of Raman spectra for the test sample described above, measuredwith a varying degree of spatial offset with respect to the Ramancollection point and the point of laser incidence on sample surface isshown in FIG. 11. For comparison, the Raman spectra of pure layersmeasured in separate measurements are also displayed. The top spectrumin FIG. 11 is that of pure trans-stilbene and the bottom spectra that ofpure PMMA. The spectrum measured with the zero offset (0 mm) representsthe Raman spectrum one would typically obtain using a conventional Ramaninstrument. It is evident that it contains an appreciable contributionfrom both the top and bottom layers of the sample and that thecontribution of the top layer gradually decreases with offset distancein the spatially offset spectra. For real applications, where a purespectrum of the bottom layer needs to be recovered, the top layer signalmight represent an unacceptable distortion to the Raman signal of alower layer. The gradual separation between the two signals is clearlyaccomplished using the SORS approach as the lateral offset between theRaman collection point and the point of probe beam incidence isincreased and is clearly observable from the illustrated data set. At adistance of >2 mm (third spectra down in FIG. 11) an order of magnitudeimprovement in the ratio of the lower over the top layers Raman signalsis achieved.

FIG. 12 shows the dependence of the absolute Raman intensities of theindividual spectra on the spatial offset. The data was obtained bynumerical fitting of two intense trans-stilbene bands at 1575, 1595,1632 and 1641 cm⁻¹ and bands at around 809, 1455, and 1728 cm⁻¹ forPMMA. The plot clearly demonstrate that as the Raman collection point ismoved sideways from the probe illumination zone, i.e. the lateral offsetis increased, the Raman signal from the bottom layer diminishes muchmore slowly than that from the top layer. This results in the overallrelative Raman intensity ratio of the bottom over the top layerimproving with increasing spatial offset as shown in FIG. 13.

To quantify the contrast improvement achieved using the method andapparatus of the present invention with respect to the test sampledescribed above, a Raman spectrum with a longer-acquisition time (1000s) at an offset of 3.5 mm was acquired. FIG. 14 shows this spectrumalong with a Raman spectrum acquired with zero offset scaled to the sameheight of trans-stilbene bands. By subtracting the pure trans-stilbenespectrum from these spectra we obtained the PMMA contributions withinthe individual spectra (see FIG. 15). By fitting these we establishedthat the contrast of-the lower layer had been improved by a factor of 15by rejecting the top layer spectral component. Another strikingobservation is that the signal-to-noise obtained using this spatialgating approach is good in comparison to alternative approaches.

The total attenuation of the Raman trans-stilbene signal by, the 1 mmPMMA layer was measured with the zero offset to be around 80. This lossof signal through the diffusion process, inevitably present also inconventional Raman spectroscopy, can be, however, effectively offsetthrough further refinements in the collection efficiency: for example byadopting the circular collection geometry shown in FIG. 10, or by usinga lower f-number and a higher throughput spectrograph.

FIG. 16 demonstrates another useful feature of the spatial gatinganalysis apparatus and method of the present invention. The analysisapparatus is capable of suppressing fluorescence in the lower layerRaman spectrum if it originates from the top layer. The plot shown inFIG. 16 gives the relative ratio of the trans-stilbene Raman signal incomparison with the fluorescence originating from the PMMA layer as wellas the fluorescence absolute intensity as a function of the spatialcollection offset. The trans-stilbene Raman intensity relative tofluorescence intensity is improved by a factor of approx. 2 with theintroduction of a 2.5 mm displacement.

In a situation where a larger separation of the data obtained fromsurface and sub-surface lavers is required than that achievable directlywithin the raw spectra, by offsetting the collection and probe launchpoints a multivariate data analysis procedure may be deployed using theanalyser 104 of FIG. 8. The data collected by SORS is particularlyamenable to multivariate data analysis because for this approach to beapplicable, the set of Raman spectra measured at various offsets isstill required. To achieve an effective numerical decomposition thenumber of spectra within the set should ideally be at least an order ofmagnitude higher than the number of layers present in the sample. Todemonstrate this a multivariate analysis of the form of principalcomponent analysis (PCA) was employed.

Approximately twenty Raman spectra acquired on the PMMA andtrans-stilbene two-layer system represented in FIG. 8 and produced usingthe SORS method and apparatus described herein were imported intoMatlab™ R11 (The Mathworks Inc., Natick, Mass.) and processed with bothbuilt in and locally written scripts. The ten largest eigenvectorsgenerated after performing a singular value decomposition on theoriginal data set were included in the PCA rotation. The pure spectra ofPMMA and trans-stilbene were not included in this dataset and nobaseline correction was performed.

Multivariate data reduction techniques are advantageous when a completeseparation of the spectral features of the surface and sub-surfacelayers is required. These data reduction techniques also provide a meansof separating spectral features from layers that may have a moderate tohigh degree of spectral overlap or where contributions of individualcomponents to spectral bands envelopes may not be known because spectraof the pure components may not be obtainable or known.

The recovered factors from the multivariate analysis are shown in FIG.17. The procedure cleanly decomposed the Raman spectra collected in thisway into the pure spectra of the two individual layers, i.e. a PMMA (toplayer) and a trans-stilbene (bottom layer). A factor for puretrans-stilbene was recovered by targeting the ca. 1595 cm⁻¹ band (pixel730) and a factor for pure PMMA was recovered by targeting the ca. 809cm⁻¹ band (pixel 80). The luminescence background factor was constructedfrom one of the original input spectra. This factor was generated usingan iterative polynomial fitting algorithm (Lieber CA andMahadevan-Jansen A 2003) typically used for baseline correction. In thiscase 100 fitting cycles using a third order polynomial were used togenerate the baseline. This baseline was used as a factor representingthe luminescence background. These three factors were then used toreconstruct the dataset with less than 3% error.

Although in the above example twenty separate Raman spectra werecollected, where a scaled subtraction of individual Raman spectra ispossible, as few as two or three spectra are required. Even withmultivariate data analysis, although it is preferred to perform theanalysis on at least a factor more than the number of components to beidentified, such analysis can often be successfully performed usingsmaller data sets of, for example, around ten spectra.

The following is the inventors' current theory for explaining theefficacy of the analysis method and apparatus described herein. Thistheory is supported by detailed Monte Carlo scattering modelling studiesalso carried out by the inventors, which yield results in very goodagreement with experiment. The variation in the relative content ofRaman signals from different layers as the collection point is spatialoffset originates from the random properties of the photon migrationeffect. The migrating photons in essence undergo a ‘random walk’ withinthe medium and the photon direction is randomised every transport lengthalong the propagation distance. When a Raman signal is collected fromthe surface of a sample at the point where the probe beam is incident,the spectrum contains a relatively large signal contribution from thetop layer due to the probe photon density being highest at the point ofsample exposure. With increasing sample depth the probe intensity fastdiminishes as the photon intensity is progressively diluted through thephoton diffusion process. Moreover, Raman light generated at deeperlayers of the sample is scattered as it propagates back to the surfaceand is subject to the same diffusion. This therefore leads to furtherdilution of the intensity of Raman spectra generated at deeper samplelayers. This effect results in a substantially larger proportion ofRaman photons generated at the sample surface being collected than thosegenerated at deeper sample lavers when a signal is collected from thesurface of a sample at the point where the probe beam is incident, incomparison to the signal that would be collected for an opticallytransparent media probed in the same geometry.

However, when Raman light is collected from a point laterally offsetfrom the point of probe beam incidence, the probe light intensity withinthe sample is becoming more equally distributed along its depth. This isbecause the incident light first had to propagate sideways through thesample from the probe incidence point to the collection area and was onits way randomised through photon diffusion. Consequently, the scatteredRaman signal collected at a position offset from the probe incidentpoint contains a higher proportion of the deeper layer signal than thatin the spectrum collected from the probe beam incidence point.

The described spatial gating analysis apparatus and method thus offersan extremely powerful, yet simple means for extracting pure Ramansignals from individual layers within diffusely scattering media. Theprobed sample depths can be well in excess of the transport length,which sets a depth limit on the conventional confocal Raman microscopy.In the above example, the transport length of the medium was estimatedto be 200 μm. Importantly, the apparatus and method can be used ‘blind’,i.e. without any prior knowledge of the chemical constituents ofindividual layers. The technique has thus ideal prerequisites forsensitive sub-surface, non-destructive probing of diffusely scatteringmaterials in both industrial and medical applications.

In situations where a sample is known to consist of only two layers ofdifferent composition (if this is not known then this information can beobtained directly from pure PCA) the method and apparatus can be used toextract the pure signals of individual layers without the involvement ofmultivariate data analysis techniques. This is possible where the twospectra of the two layers each include an identifiable band or bandsthat do not overlap. In this situation a simple scaled subtraction canbe used to separate the spectra of each of the individual layers fromeach other. In this process one Raman component is eliminated by ascaled subtraction of two spectra measured with two different spatialoffsets cancelling out one or other spectral component in the process.The results of this simple extraction procedure are shown in FIG. 18.The spectra used in the analysis were measured with a zero and a 2 mmoffset. The result is clearly satisfactory, although the applicabilityrequires the above conditions to be satisfied. In contrast, the PCAanalysis described above can be used in circumstances where there is noknowledge of the compositions of the different layers of a sample.

Thus, it will be apparent that it is not in all cases essential for acomplete Raman spectrum to be generated with the present invention.Where there is some knowledge of the materials involved or thecompositions to be detected, detection of individual Raman spectralfeatures using, for example, one or more band pass filters is alsoencompassed by the SORS method and apparatus described herein.

The exact degree of the ‘suppression’ or separation of two layers ingeneral situations depends on a variety of parameters. These parametersinclude the thickness of the top layer, that of the underlying matrix,the probe beam diameter, the exact collection geometry, the wavelengthof the probe light used and the transport length of the medium. Fornon-invasive sub-surface probing, as a rule of thumb, it is believedthat the ideal offset should be on the scale of the thickness or severalthicknesses of the overlying medium. Also, for the technique to beeffective the beam diameter should be smaller than the thickness of thetop layer. In general terms the thinner the top layer is and the thickerthe underlying matrix is favours a better spectral separation of the twocomponents.

The use of probe light of particular wavelengths is not critical to thisinvention. The choice of probe wavelength is essentially a trade offbetween depth penetration, which improves with longer wavelength, anddetector quantum efficiency, which is higher at shorter wavelengths. Asmentioned earlier, the detector 3 used herein is a backilluminated deepdepletion CCD detector based on silicon technology. This detector isselected as it has the best sensitivity and signal-to-noise ratio ofthose that are currently available, but alternatives can be used. Longerwavelengths avoid exciting H₂O modes in Raman spectra, but the cut-offlimit for Si detection is 1.1 μm. InGaAs detectors can be used at longerwavelengths, but these have currently reduced sensitivity.

With the method and apparatus of the present invention, substantiallypure Raman spectra can be retrieved from depths well in excess of thoseaccessible with conventional confocal microscopy. Moreover, the presentinvention has the advantage that it is compatible with the use of cwlasers beams and is suited to remote monitoring in both industrial andmedical applications. Thus the method and apparatus are well suited tonot only biomedical applications, where monitoring sub-surface tissuelayers normally would require destroying surface tissue, but also manyindustrial analytical applications such as catalysts, food, and polymersresearch applications. The present invention may be used to detectcontamination of food during manufacture or the deleterious breakdown offood in storage as well as the stability of stored pharmaceuticals inall cases without any contact with the sample.

1. A method of measuring sub-surface Raman scattering through adiffusely scattering sample, comprising: (a) supplying incidentradiation at one or more entry regions on a surface of the sample; (b)collecting light scattered within the sample, from a collection regionon the surface, wherein one or more of the one or more entry regions arelocated around the collection region; (c) detecting, in the collectedlight, one or more Raman features spectrally related to the incidentradiation; and (d) deriving, from the one or more Raman features, one ormore characteristics of a sub-surface region of the sample.
 2. Themethod of claim 1, wherein at least one of the one or more entry regionslocated around the collection region forms an annulus around thecollection region.
 3. The method of claim 1, wherein said collectionregion is a central collection region surrounded by the one or moreentry regions located around the collection region.
 4. The method ofclaim 3, wherein the one or more entry regions located around thecollection region are multiple annular entry regions having a range ofradii.
 5. The method of claim 1, wherein the one or more entry regionslocated around the collection region include a variable or fixed radiusentry region located around said collection region.
 6. The method ofclaim 1, wherein said collection region is a central collection regionand at least one of the one or more entry regions located around thecollection region is a broken or incomplete annular region includingmultiple separate illumination regions distributed at similar distancesfrom said central collection region.
 7. The method of claim 4, whereinsaid multiple annular entry regions are broken or incomplete annularregions which surround said central collection region.
 8. The method ofclaim 7, wherein a broken annulus includes multiple separateillumination regions distributed at similar distances from said centralcollection region.
 9. The method of claim 1, wherein the incidentradiation is supplied by a plurality of optical fibres distributedaround one or more collection optical fibres used to collect said lightscattered within the sample.
 10. The method of claim 1, wherein saidcollection region is a central collection region and the one or moreentry regions located around the collection region include an annularillumination region surrounding said central collection region, therebyreducing a localised intensity of incident radiation required togenerate a given number of Raman photons.
 11. The method of claim 1,wherein step (c) comprises spectrally dispersing the collected light toform a Raman spectrum for each of said one or more entry regions. 12.The method of claim 1, wherein incident radiation is supplied to aplurality of said entry regions each located at a different distancefrom said collection region, and the step of collecting comprisescollecting light, separately for each entry region, at said collectionregion on the surface of the sample.
 13. The method of claim 12, whereinthe step of detecting comprises detecting, separately for each differententry region, one or more Raman features spectrally related to theincident radiation.
 14. The method of claim 13, wherein the step ofderiving includes associating the Raman features associated withradiation from each of said entry regions with a different depth ordistributions of depth within the sample.
 15. The method of claim 13,further comprising combining the Raman features for the plurality ofentry regions to preferentially select for a particular depth or rangeof depths.
 16. The method of claim 1, wherein the step of collectingcomprises collecting light from said collection region which is spacedby different distances from multiple entry regions.
 17. The method ofclaim 1, wherein the one or more entry regions located around thecollection region surround said collection region.
 18. The method ofclaim 17, wherein each entry region located around the collection regionis an annulus, and for each annulus, incident radiation is delivered toat least 10%, and more preferably at least 25% of the surface area ofthe annulus.
 19. The method of claim 1, further comprising adjustingoptics disposed in the path of the incident radiation to adjust thedistance between the collection region and an entry region.
 20. Themethod of claim 1, further comprising disposing one or more mirrorelements adjacent to the sample surface outside the one or more entryregions to reflect light back into the sample.
 21. The method as claimedin claim 13, wherein Raman features for at least two entry regions arecollected and are analysed using a scaled subtraction, whereby Ramanfeatures for a sub-layer of the sample are identified.
 22. The method asclaimed in claim 1, wherein the Raman features for the chemicalcomposition of the surface of the sample are known and the Ramanfeatures are analyzed by scaled subtraction of the known Raman featuresfrom the Raman features of the collected light.
 23. The method asclaimed in claim 13, wherein the Raman features for said plurality ofentry regions are analyzed using multivariate data analysis.
 24. Themethod as claimed in claim 1, wherein the plurality of Raman spectra areanalysed using principal component analysis.
 25. The method as claimedin claim 1, wherein the sample is irradiated at two or more differentwavelengths and the collected light is a combination of Raman featuresand fluorescence and wherein the method comprises the further step ofextracting the Raman features from the collected light.
 26. The methodas claimed in claim 1, wherein the collected light is spectrallydispersed using a spectrometer in combination with a CCD camera.
 27. Themethod as claimed in claim 1, wherein the incident radiation isgenerated using a continuous wave laser.
 28. The method as claimed inclaim 7, wherein the incident radiation comprises two or more separatewavelengths and is generated by one or more lasers.
 29. The method asclaimed in claim 1, wherein the incident radiation is at a wavelengthof >200 nm and <2000 nm.
 30. The method of claim 1, wherein one of saidentry regions and said collection region are at the same place orsubstantially overlapping.
 31. The method of claim 1, wherein said oneor more entry regions and said collection region are non-overlapping.32. Apparatus for measuring sub-surface Raman scattering through adiffusively scattering sample, comprising: a light source arranged tosupply incident radiation at one or more entry regions on a surface ofthe sample; a collector arranged to collect light scattered within thesample, from a collection region on the surface, wherein one or more ofthe one or more entry regions are located around the collection region;a detector arranged to detect, in the collected light, one or more Ramanfeatures spectrally related to the incident radiation; and an analyzerconfigured to derive, from the one or more Raman features, one or morecharacteristics of a sub-surface region of the sample.
 33. The apparatusof claim 32, wherein at least one of the one or more entry regionslocated around the collection region forms an annulus around thecollection region.
 34. The apparatus of claim 32, wherein saidcollection region is a central collection region surrounded by the oneor more entry regions located around the collection region.
 35. Theapparatus of claim 34, wherein the one or more entry regions locatedaround the collection region are multiple annular entry regions having arange of radii.
 36. The apparatus of claim 32, wherein the one or moreentry regions located around the collection region include a variable orfixed radius entry region located around said collection region.
 37. Theapparatus of claim 32, wherein said collection region is a centralcollection region and at least one of the one or more entry regionslocated around the collection region is a broken or incomplete annularregion including multiple separate illumination regions distributed atsimilar distances from said central collection region.
 38. The apparatusof claim 34, wherein said multiple annular entry regions are broken orincomplete annular regions which surround said central collectionregion.
 39. The apparatus of claim 32, wherein the incident radiation issupplied by a plurality of optical fibres distributed around one or morecollection optical fibres used to collect said light scattered withinthe sample.
 40. The apparatus of claim 32, wherein said collectionregion is a central collection region and the one or more entry regionslocated around the collection region include an annular illuminationregion surrounding said central collection region, thereby reducing alocalised intensity of incident radiation required to generate a givennumber of Raman photons.
 41. The apparatus of claim 32, wherein thelight source is arranged to supply incident radiation to a plurality ofsaid entry regions each located a different distance from saidcollection region, and the collector is arranged to collect light,separately for each different entry region or distance, at saidcollection region.
 42. The apparatus of claim 41, wherein the detectoris arranged to detect, separately for each different entry region ordistance, one or more Raman features spectrally related to the incidentradiation.
 43. The apparatus of claim 42, the analyzer being configuredto combine the Raman features for the plurality of entry regions ordistances, to preferentially select the features for a particular depthor range of depths.
 44. The apparatus of claim 32, wherein the detectorcomprises a spectrometer arranged to spectrally disperse the collectedlight to separate out said Raman features associated with radiationirradiated at said one or more entry regions.
 45. The apparatus of claim32, wherein the detector comprises one or more filters arranged toselectively transmit said Raman features.
 46. The apparatus of claim 32,wherein the one or more entry regions located around the collectionregion are provided by an annular region surrounding the collectionregion, said apparatus further comprising an optics arrangementconfigured to controllably adjust the diameter of the annular region.47. The apparatus of claim 32, further comprising a masking devicearranged to controllably adjust the spacing between said one or moreentry regions and the collection region.
 48. The apparatus of claim 32,further comprising one or more mirror elements disposed adjacent to thesample surface outside said one or more entry regions so as to reflectlight back into the sample.
 49. The apparatus as claimed in claim 43,wherein the analyser is configured to perform a scaled subtractionbetween Raman features for at least two entry regions or distances. 50.The apparatus as claimed in claim 43, wherein the analyser is configuredto perform multivariate data analysis on the Raman features for saidplurality of entry regions or distances.
 51. The apparatus as claimed inclaim 43, wherein the analyser is configured to perform principalcomponent analysis on the Raman features for said plurality of entryregions or distances.
 52. The apparatus as claimed in claim 32, whereinthe light source comprises optical fibres arranged around the collector.53. The apparatus as claimed in claim 32, wherein the optical fibres arearranged in a plurality of concentric circles around the collector. 54.The apparatus as claimed in claim 32, wherein the light source is acontinuous wave laser.
 55. The apparatus as claimed in claim 32, whereinthe detector comprises a CCD camera.
 56. The apparatus of claim 32,wherein one of said entry regions and said collection region are at thesame place or substantially overlapping.
 57. The apparatus of claim 32,wherein said one or more entry regions and said collection region arenon-overlapping.
 58. A method of detecting one or more sub-surfaceproperties of a sample, comprising: directing probe light into saidsample through one or more surface entry regions; collecting a portionof said probe light following scattering through said sample from saidone or more entry regions to a surface collection region disposed withinat least one of said one or more surface entry regions; detecting Ramanspectral features in said collected probe light; and analyzing saidRaman spectral features to derive said one or more sub-surfaceproperties.
 59. The method of claim 58, wherein the step of directingcomprises directing probe light into said sample through a concentricplurality of said surface entry regions; and wherein the step ofanalyzing comprises combining the Raman spectral features for theplurality of surface entry regions to select for depth in said one ormore sub-surface properties.